1. Field of the Invention
The present invention relates to a process for the preparation of expandable beads.
In more detail, the present invention relates to a process for the precipitation of expandable beads constituted by blends of poly(phenylene-ether) and of a vinyl aromatic polymer.
2. Discussion of the Prior Art
As known the poly(phenylene-ether) resins can be mixed with the vinyl aromatic polymers to form thermoplastic compounds that are extrudable and moldable in moulded articles having high thermal resistance, good impact resistance and hydrolytic stability, and good dimensional stability.
These compounds can also contain flame retarding agents, reinforcing agents and also different quantities and types of expanding agents.
The poly(phenylene-ether) resins and the methods for their production are widely described in the patent literature and to give an example we remind to the U.S. Pat. Nos. 3,306,874, 3,306,875, 3,257,357, 3,257,358, 3,337,501 and 3,787,361. The blends of poly(phenylene-ether) with polymers of vinyl aromatic monomers, included polystyrene, high impact polystyrene and the co- and terpolymers of the styrene, are described in the U.S. Pat. No. 3,383,435.
The foamable blends of poly(phenylene-ether) and vinyl aromatic polymers are known too. These blends are generally used to produce structural foams using molds and techniques analogous to the injection molding process.
From the U.S. Pat. No. 4,727,093 a process is known to obtain low density particles of poly(phenylene-ether) and vinyl aromatic polymers blends. This process consists of incorporating in the already formed granules of the poly(phenylene-ether) and a vinyl aromatic polymer blends, a liquid volatile expanding agent, such as an C.sub.4 -C.sub.7 aliphatic hydrocarbon or C.sub.1 -C.sub.2 fluorocarbon that is able to be absorbed by the blend and that vaporizes at a temperature lower than the glass transition (Tg) one of the blend. To this purpose, the granules as such, or better in aqueous suspension, are treated with the expanding agent in an atmosphere saturated of the mentioned agent for a time sufficient to incorporate such a quantity of it to obtain successively the desired expansion.
After this treatment, the granules are submitted to an expansion process to obtain the low density product. The expansion process takes place by heating at a temperature near the Tg of the blend, preferably carried out by pressurized steam.
These processes for the incorporation of the expanding agent cause different problems that influence both the aspect and the properties of the particles.
As a matter of fact, the known processes bring to a dishomogeneous absorption of the expanding agent by the particles, that after the expansion, cause an unperfect cellular structure (glassy-structure) and also an unperfect and unacceptable diffusion of the expanding agent in the blend. For this reason and because of the unsufficient penetration of the expanding agent, the presence of cores inside the expanded particles is observed.
Moreover the expandable particles obtained by this incorporation method, generally require a quite long maturation to cause a sufficient diffusion of the expanding agent, before their preexpansion.
Moreover the expandable particles obtained by this procedure have only a roughly spherical shape and their diameter is such that it is not possible to use them to mold manufacturers having complex geometrical shapes obtaining a good superficial aspect.